The strain rate dependent plastic flow behavior of zirconium and its alloys
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I N r e c e n t y e a r s it h a s b e c o m e i n c r e a s i n g l y e v i d e n t that z i r c o n i u m and some of its alloys exhibit an a n o m a l o u s m e c h a n i c a l b e h a v i o r over the t e m p e r a t u r e range of a p p r o x i m a t e l y 200 ~ to 400~ The o b s e r v e d p h e n o m e n a in this t e m p e r a t u r e r a n g e include the y i e l d - p o i n t effect and the flow s t r e s s i n c r e a s e after i n t e r r u p t e d loading in t e n s i o n t e s t i n g , 1- lo the d e c r e a s i n g c r e e p r a t e with i n c r e a s i n g t e m p e r a t u r e , 11-13 and the d e c r e a s ing t e n s i l e ductility with both i n c r e a s i n g t e m p e r a t u r e 12'14 and d e c r e a s i n g s t r a i n r a t e . Is Many of t h e s e p h e n o m e n a have b e e n r e l a t e d to some form of s t r a i n aging, and a v a r i e t y of m e c h a n i s m s have b e e n proposed. F o r e x a m p l e , the effects have b e e n a s s o c i a t e d with the p r e c i p i t a t i o n of tin in Z r - 1 . 5 Sn alloy, a with h y d r o g e n s e g r e g a t i o n in Z i r c a l o y - 2 , 5 with the C o t t r e l l locking of d i s l o c a t i o n s by i m p u r i t y a t m o s p h e r e , is or with the other d i s l o c a t i o n - i n t e r s t i t i a l i n t e r a c t i o n s , such a s with oxygenfl '9 The d y n a m i c d i s l o c a t i o n m o d e l for y i e l d i n g also has b e e n applied to explain the y i e l d point b e h a v i o r . 6 Other r e l a t e d a n o m a l i e s in these a l l o y s have a l s o b e e n r e p o r t e d . It was found that the a p p a r e n t a c t i v a tion e n e r g y in c r e e p i n c r e a s e d a b r u p t l y followed by a s h a r p drop at about 350~ 13'16 T h i s was a g a i n a t t r i b u t e d to a s o l u t e - d i s l o c a t i o n i n t e r a c t i o n . Another notable c h a r a c t e r i s t i c i s that of the s h a r p l y l o w e r s t r a i n r a t e s e n s i t i v i t y of the flow s t r e s s in the t e m p e r a t u r e r a n g e of 300 ~ to 400~ in pure z i r c o n i u m , 7 Z i r c a l o y - 2 , 3 and Zircaloy-4.17 S i m i l a r m i n i m a in s t r a i n rate s e n s i t i v i t y w e r e also o b s e r v e d in single c r y s t a l s both of Zircaloy-218'19 and z i r c o n i u m oxygen a l l o y s .20,21 F o r some of the widely used complex c o m m e r c i a l a l l o y s like Z i r c a l o y - 2 , this a n o m a l o u s b e h a v i o r has not yet b e e n a d e q u a t e l y c h a r a c t e r i z e d . The m u l t i p l i c i t y of alloying e l e m e n t s in addition to t i n and oxygen in such a l l o y s is the r e a s o n for this lack of D. LEE is Metallurgist,General Electric Research & Development Center, Schenectady, N. Y. Manuscript submitted September 30, 1969. METALLURGICALTRANSACTIONS
u n d e r s t a n d i n g . It is t h e r e f o r e i m p o r t a n t that the n a t u r e of t h i s a n o m a l o u s b e h a v i o r be identified and a l s o that the f l o w - s t r e s s b e h a v i o r up to and i
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